Carbon fibers are generally produced in the form of filaments or a tow (a bundle of several hundreds to several hundred thousands of filaments). The filaments or tow are usually used in the form of a sheet or tape produced by disposing the filaments in one direction and adhesion-processing them, woven or knitted fabric, etc. Alternatively, they may be used by cutting them into a length of several mm to several tens of mm. During processing steps for obtaining these fiber products, the use of carbon fibers in an as-produced form is liable to cause fluffing, leading to inferiority in handling. In order to prevent carbon fibers from fluffing, a sizing agent is usually applied to the carbon fibers to increase their bundling properties.
Sizing agents for carbon fibers are classified into two types. One type is a solution type as described in, for example, U.S. Pat. Nos. 3,806,489, 3,914,504 and 3,837,904. The solution type is comprised of an organic resin such as polyvinyl alcohol, vinyl acetate polymer, acrylic polymer, polyurethane, epoxy resin or polystyrene dissolved in an organic solvent. The other type is an emulsion type as described in, for example, U.S. Pat. No. 4,219,457, which comprises the above-described organic resin dispersed in water with the aid of an emulsifier. The solution type sizing agents require a large amount of organic solvent, and hence they are disadvantageous from the standpoints of economy, safety, and hygiene. Accordingly, emulsion type sizing agents are ordinarily used.
When depositing an emulsion type sizing agent onto carbon fibers, agents which have a solid concentration of about 0.1% to about 15% are employed in some cases. Sizing agents having such a low solid concentration have inferior emulsion stability (or emulsification stability). Furthermore, when applying emulsified particles onto carbon fibers having a low surface energy by using an emulsion type sizing agent for sizing, application specks are often created. Therefore, only fiber bundles with poor bundling properties are obtained. Furthermore, the heat stability of the sizing agent is decreased by the effects of the emulsifying agent used. This leads to deterioration of the physical properties of a carbon fiber-reinforced composite material obtained. These effects are caused by using carbon fibers which have been treated with these types of sizing agents, and, for example, a thermosetting or thermoplastic resins as a matrix material.